1. Field of the Invention
This invention relates generally to series/parallel LED drive systems, and more particularly to methods of controlling the power supply which provides the line voltage for parallel-connected LED strings.
2. Description of the Related Art
LED lighting strategies may employ LEDs driven in series, parallel, or both. LEDs driven in series by definition all share the same current. If all LEDs share the same current, ideally the brightness of the LEDs will be matched. Some applications require a number of LEDs to be driven with matched brightness, and so connecting the LEDs in series accomplishes the task. A problem can arise, however, if a very large number of LEDs must be driven in series. The series-connected LEDs are powered by a line voltage necessary to provide the necessary current; however, finding line regulators able to support the large line voltage needed for a high LED count series string may be difficult or cost prohibitive.
LEDs may also be arranged in parallel-connected ‘strings’, each of which is driven by a current source or (most commonly) a current sink circuit. But brightness matching between the parallel-connected LEDs is limited by the imperfect matching of the drive circuits, which can vary widely depending on the choice of sink implementation. A parallel LED configuration does have the advantage of typically requiring a lower line voltage than does a series configuration, which may be a benefit in some applications. Also, in some applications LEDs are connected in parallel because different currents need to be driven through the LEDs.
Due to the issues noted above, the best approach may be a compromise between the series and parallel solutions: a “series/parallel” solution. A cost-effective compromise employing a series/parallel solution is shown in FIG. 1. Here, each series LED string 1, 2, 3 has its own independent current sink circuit 4, 5, 6, but all series strings share a common line voltage Vline, which is provided by a voltage regulator 7. The voltage on the current sink circuits (VD0, VD1, VD2) is generally set to be equal to the maximum voltage that a string of LEDs might have on its anode connection. This can be arranged by means of a “minimum” circuit 8, which receives the voltages on each of the current sinks and outputs the minimum voltage of the group. An error amplifier 9 receives the minimum voltage and a reference voltage VDdesired at respective inputs, and provides an output Verr to the feedback input of voltage regulator 7 such that the current sink circuit with the minimum VD voltage operates at a desired target voltage equal to VDdesired.
Conventionally, the components (such as voltage regulator 7) which generate common line voltage Vline are integrated with the other elements of the LED drive system to form a complete LED controller IC. However, including this power supply functionality within the LED controller IC may be costly and require an unacceptably large amount of die area.